You hadn't mentioned temperature - at what temp are you keeping them currently? If birds are too hot or too cold, they will not eat readily and will die off. Their lack of eating can also predispose them to other things of which they can get sick.
What are their droppings like? What kind of bedding are they on, what type of brooder set up, are they near older birds, is it particularly hot there?
I hate to say it's one thing or another based on just two bits of information. It might still be, but there's a much bigger picture and I think it would help you if you gave us some more information.
There might be other things happening here.